A coastal zone oil spill model: Development and sensitivity studies

Abstract

Oil spill trajectory and fates models typically follow a surface slick until it contacts a coastline, at which time the simulation ceases. The coastal zone oil spill (COZOIL) model described here is designed to simulate oil spill fates both before and after a coastal contact. Multiple discrete batches of oil (spillets) are used to represent the surface slick. Spillets are circular while offshore but become elliptical upon contact with the shoreline. Onshore-offshore foreshortening is governed by a balance between wind stress and gravity spreading forces, and results in alongshore spreading of the spillet. Evaporated hydrocarbons are accumulated from all sources during the simulation, with no spatial representation. Entrained oil offshore is represented by discrete particles which maybe advected by the local currents. Inside the surf zone, entrained oil takes on a continuous representation, discretized within individual alongshore grid cells. Transport in the surf zone is governed by a classical radiation stress formulation. Incorporation of water into surface oil (emulsification) is simulated offshore. De-emulsification (de-watering) is allowed to occur for oil which is on the foreshore or backshore. Oil coming ashore may be deposited on the foreshore or the backshore, or carried into coastal indentations (lagoons, ponds, or fjords). Each of the seven shoreline types represented in COZOIL is characterized by a unique set of parameters, including grain size, porosity, and a maximum oil thickness which the foreshore can retain. Oil on the foreshore penetrates into the underlying sediments at a rate dependent on sediment grain size and oil viscosity. Oil may also be carried into the beach groundwater system by wave overwash. Reflotation of surface oil occurs during rising tides. The model is inherently deterministic with respect to results of any single simulation. Stochastic oil distribution estimates are produced by combining the results of multiple simulations, each of which may be driven by a separate weather scenario.